Vegetarianism proved too hard to swallow for early humans, with new research bolstering evidence that our ancestors likely veered from this lifestyle around 2.6 million years ago in favour of raw red meat and starch-rich plants.

original paleo diet KEY POINTS

Key Points

Meat and tools responsible for evolution of smaller chewing-related features on humans, research suggests

Authors of research say there is no one optimal diet for everyone today

The study, published in the journal Nature, suggests that meat and tools — and not the later advent of cooking — freed early humans to evolve smaller chewing-related features, such as smaller teeth and smaller, shorter faces.

These, in turn, might have paved the evolutionary way for improved speech, thermoregulation and even the development of a bigger brain.

"No one knows for sure why hominins started to eat more meat around 2.6 million years ago, but there is abundant evidence for this behaviour, including stone tools and cut marks on bones," said Katherine Zink, the lead author of the study from Harvard University's Department of Human Evolutionary Biology.

"The most common explanation is climate change. During this period, Africa became more open grassland, with more antelopes and other herbivores."

This early paleo diet did not include any seafood.

"The ability to fish appears to have come much later in human evolution. Shellfish consumption also appears to be relatively recent," she said.

How did humans develop bigger brains while teeth got smaller?

The study was prompted by a paradox that has puzzled researchers for years.

By the time of Homo erectus about two million years ago, humans had evolved bigger brains, bodies and presumably appetites, but their teeth and gut were smaller than those of earlier ancestors.

Dr Zink and co-author Daniel Lieberman suspectedS that mechanical processing of food, as well as the addition of red meat into the diet, could explain the seeming paradox.

To test this, the researchers fed adult test subjects samples of goat meat (one of the chewier red meats) and jewel yams, carrots and beetroots (to stand in for the starchy plant storage organs that early humans ate). They measured the muscular effort required for chewing and how well the food was broken up before swallowing.

The scientists found that pounding the plant material with stone tools and eating a diet composed of one-third sliced red meat reduced the need to chew by 17 per cent and lowered needed force by 26 per cent.

Dr Zink and Dr Lieberman point out that prior archaeological evidence indicates early humans fabricated stone tools by 3.3 million years ago, but did not learn to control fire until around 1 million years ago. Evidence for cooking on a regular basis dates to at least 500,000 years ago, long after evolutionary selection for smaller human teeth began.

As for why there was (and still is, among African foragers) dietary emphasis on underground starchy plants, the researchers explained that humans emerged at a time when the environment in Africa was drying and juicy, sweet fruits were less plentiful.

"During this time," Dr Lieberman said, "many of the forests thinned and transitioned into more open grasslands".

He and Dr Zink added that starchy underground plants are much more plentiful in grasslands than the fruit/tree-based foods of most chimpanzees, and possibly also of the last common ancestor of chimps and humans.

Should paleo dieters take note?

Anthropologist David Strait of Washington University in St. Louis said the study inspires many follow-up questions, such as how tooth mechanics affect the processing of meat, tubers, bulbs and corms. Dr Strait also wonders if particular tooth shapes, sizes and configurations are better able to resist being fractured when eating such foods.

So should dieters take their cue from this original paleo diet of red meat and tubers?

The authors caution against the idea that there is an optimal diet now for everyone. Later humans in places like Europe and Asia ate tons of fish, for example, with some living to ripe old ages.